This invention relates to processes designed to treat mono ethylene glycol (MEG) used in the oil and gas industry, especially in offshore locations, to control hydrates formation. More particularly, the invention relates to MEG reclamation processes which are designed to remove salts from a wet MEG feed stream.
In the oil and gas industry, dry (lean) MEG is used to control the formation of hydrates within the produced stream. The now wet (rich) MEG is, in turn, dried by way of a MEG reclamation process so the MEG can be used again in hydrate control. However, the lean MEG cannot be recovered by simply distilling the rich MEG and water because the rich MEG is commonly loaded with salts such as calcium chloride and, commonly, lesser amounts of other divalent metal salts like magnesium, barium and strontium chlorides. The salts have to be removed before the MEG and water are separated. If the salts are not removed prior to the MEG/water separation, the salts accumulate in the process equipment, eventually leading to process failure. This is especially problematic in processes whose design does not include a divalent metal salt removal step
In processes that do include a divalent metal salt removal step, the divalent metal is usually removed by reacting it with precipitant agent such as sodium carbonate (soda ash), potassium carbonate or sodium oxalate. The resulting insoluble salt precipitates and is removed as solid crystals. Because the most common divalent metal present in rich MEG is calcium and the most common precipitating agent is sodium carbonate, the following descriptions are based on calcium removal as representative of divalent metals removal and sodium carbonate as representative of precipitating agents.
In a typical process having a calcium removal step, the calcium in the rich MEG will be reacted with sodium carbonate (soda ash) to form insoluble calcium carbonate and soluble sodium chloride. The insoluble calcium carbonate precipitates and is removed as solid crystals. The soluble sodium chloride remains in solution and is separated from the rich MEG by flashing the rich MEG under vacuum and at temperature. The vaporized water and MEG are then separated by partial condensation in what is commonly termed a “distillation” tower.
Flashing of the solvents (vaporized water and MEG) for the remaining salts (mostly sodium chloride) turns the salts into crystals that drop into a salt-saturated hot MEG solution which, in turn, is used as the heat transfer medium to drive the flashing process. Calcium presence in this step can promote severe viscosity problems and hinder proper settling of the newly formed crystals.
Calcium removal with soda ash treats the rich MEG stream at about 180° F. and needs about 15 minutes of residence time for the reaction to proceed and form reasonable size calcium carbonate crystals for decanting and removal. However, experiments conducted by the inventor have shown that calcium carbonate crystal growth, final size and reaction speed are normally favored by calcium chloride concentrations typically far above the concentration normally found in the rich MEG stream (usually less than 2,000 ppm). Lower concentrations of calcium in the precipitator inlet favor small size crystal formation, which in turn increases the difficulty of removal and residence time for the reactions to occur. The temperature requirement usually entails the use of heat exchangers to heat up the feed to the reaction temperature and may require more real estate than available, especially in offshore locations.
A need exists for a MEG reclamation process that removes calcium more effectively especially when the initial design did not include a calcium removal step (which is then found necessary later on) and intensifies the calcium removal process to make better use of limited space and existing equipment. Notwithstanding, the process can also be used for new designs to provide for a more compact divalent metal removal step than the current art.